Devices to generate a magnetic force, such as are present in magnetic force drives of switches, are used, for instance, in association with position switches in both the industrial and the private field. They are used to make a danger area of a dangerous machine or production plant safe.
Position switches are also used in safety engineering, plant engineering, automation engineering and building services engineering. In this environment, for instance doors, flaps or other movable objects which are used for access or approach to parts of the machine or production plant must be made safe, meaning in detail that the relevant object is detected in the secure position and if appropriate locked in the secure position by a tumbler.
Position switches, particularly safety switches, are used for safe locking of protective doors, where for plant engineering or physical reasons opening the protective doors does not result in the dangerous potential being immediately switched off, but for instance, because of overrun of large drives, the dangerous potential remains until complete standstill. Such protective doors must be protected reliably against opening.
To lock and unlock the position switch, a tumbler is provided. The tumbler contains a device to generate a magnetic force. Depending on the version, the magnetic force or an elastic force can be provided to execute the locking movement. A position or safety switch with tumbler accordingly contains mostly elements which generate magnetic force and elastic force. The elastic force or elastic forces counteract the magnetic force, and depending on the type of locking either increasing or reducing the magnetic force results in locking the tumbler.
Such devices to generate a magnetic force, which are also called magnet systems or magnetic drives, must fulfill various conditions. To overcome the counterforce of the circuit element, as high a force budget as possible with as large a switching path as possible should be available. In this case a high force must be available, particularly in the end positions of the armature. Because the armature usually has two end positions and the coil force rises exponentially depending on the movement of the armature when the tumbler is switched on, at least one end position of the armature has a small magnetic force. This results in comparatively small forces at the start of the movement, and these affect the switching behavior of the tumbler disadvantageously. Additionally, as small as possible a design of the position or safety switch is wanted, as is a low power consumption.
In FIG. 1, the graphs G1 and G2 are shown. G1 shows the force of a switching unit depending on the switching path S. G1 rises with the deflection S of the armature, which moves from the idle position into the locking position. Usually, in the process of actuating a circuit element a series of springs and elastic forces is involved, so that during the counterforce increase discontinuous changes occur.
It is problematical that the coil force precisely near the idle position of the armature is relatively small, mostly zero. G2 shows clearly that the coil force begins at the origin of the graph and would only be able to overcome the counterforce of the switching unit after a certain startup distance of the armature. In practice, in this case the result would not be a switching event, because the equilibrium of forces at the origin would be in favor of the counterforce. Until now, this has been got round by a short-term overcurrent to the coils which are included in the magnetic drive, so that a short-term magnetic force increase is generated to overcome the counterforce. This has a negative effect on the lifetime of the magnetic drive.
From EP 0 977 228 B1, a magnetic force drive with a high force budget on the basis of a short-term overcurrent is known. It is intended for use in a position switch.